So, I thought I'd put together a thread on how I attach the original sling on an 1873 trapdoor Springfield. Bear in mind that it's a little different with the earlier style sling with the standing loop instead of the button. The earlier style has a smaller hook too which makes it easier to thread it through the sling swivels.
With the later button style sling with the double claw hook, the first thing I do is thread the hooked end through the upper sling swivel from the outside to the inside.
Then I thread on the keepers
If I'm setting the sling up for parade or inspection, I put them both on both sides of the sling
Then I run the hook end through the bottom sling swivel from inside to outside.
Then I thread the button end of the hook through the bottom sling from the ouside of the sling to the inside and underneath the hooked end of the sling and button it.
Finally, I pull all the slack out of it that I can and hook it into itself and adjust the keepers.
If I were adjusting it for "Sling Arms" in the field on a march, I'd omit running the hooked end through the lower sling swivel and hook it into itself above the lower sling swivel at a length that I find to be comfortable for carry